Michael reactions in which base-catalysed addition of activated methylene to polarized double bond are utilized in the crosslinking of resinous compositions for coating use. Theses compositions are advantageous in that the Michael reaction gives a chemically stable crosslinking bond without forming any reaction by-product.
Typical examples of prior art include the following patent literature:
U.S. Pat. No. 4,408,018; PA1 U.S. Pat. No. 4,602,061; PA1 U.S. Pat. No. 5,017,649; PA1 U.S. Pat. No. 5,084,536; PA1 EP-A-0448154; PA1 JP-A-01/121341 (EP-A-0310011); and PA1 JP-A-01/204919 (EP-A-0326723).
Typical examples of activated methylene compounds used in the Michael reaction are derivatives of acetoacetic acid, cyanoacetic acid and malonic acid. For use in the crosslinking reaction of resins or polymers, the Michael donor must be a compound or polymer having a plurality of activated methylene groups in the molecule. Acrylic monomers having an acetoacetoxy group can be synthesized relatively easily by reacting diketene with a hydroxylalkyl acrylate or methacrylate such as 2-hydroxyethyl acrylate (HEA) or 2-hydroxylethyl methacrylate (HEMA). Activated methylene-containing acrylic polymers used in the prior art are either polymers of acetoacetylated hydroxyalkyl (meth)acrylate monomers or acetoacetylated acrylic polyols produced by reacting diketene with an acrylic polyol polymer.
Because malonic acid is a dibasic acid, malonate groups can be incorporated into a polyester chain to produce a polymeric Michael donor. U.S. Pat. No. 4,602,061 discloses a liquid two-component coating composition comprising (a) an oligomeric or polymeric malonate compound such as polyurethanes, polyesters, polyacrylates, epoxy resins, polyamids or polyvinyl resins containing malonate groups, either in the main chain or the side chain or in both, (b) an .alpha., .beta.-ethylenically unsaturated carbonyl compound and (c) a basic catalyst.
The malonate group-containing polyacrylate resin disclosed therein is a reaction products of the above-mentioned malonate polyester resin first with a diisocyanate and then with HEA or HEMA.
JP-A-03206012 discloses a malonate group-containing acrylic monomer and polymers thereof for use as odorless, low irritating and low water absorbing dental materials. Corresponding acrylate monomers and polymers containing higher homologs such as succinate or adipate are also disclosed. The use of these monomers and polymers in combination with a Michael acceptor and a catalyst are not disclosed therein.
It has been discovered that resinous compositions containing a malonate group-containing acrylate polymer, a Michael acceptor and a catalyst are advantageous in terms of weatherability over corresponding compositions containing an acetoacetate group-containing acrylate polymer when used for coating purposes.